1. Field of the Invention
The present invention relates to a seat belt device for an automobile, and, in particular, to an overlock prevention mechanism for a vehicle acceleration sensor on a retractor in the seat belt device.
2. Description of the Prior Art
A seat belt device for an automobile utilizes a webbing to restrain and protect the passenger when an emergency affecting the vehicle such as a collision and the like occurs.
The webbing is wound up on a retractor and stored. When the seat belt is put on, the webbing is pulled out of the retractor and secured by inserting a tongue on the end of the webbing into a buckle.
The webbing is installed on the retractor through a comparatively weak spring force. Accordingly, the webbing is wound up or pulled out against the weak resistance of the spring of the retractor which pulls the webbing, so that while using the seat belt, the passenger would not have oppressive sensation from the seat belt.
To keep this condition, it is preferable that the retractor normally be left unlocked. However, in case of emergency involving a collision or the like, it is necessary that the webbing be locked to restrain and protect the passenger. For this reason, a vehicle acceleration sensor is built into the retractor for locking the webbing when an increase in acceleration greater than a predetermined rate is detected by the vehicle acceleration sensor.
FIG. 1 shows a conventional vehicle acceleration sensor in the locked state for a webbing (not shown). The vehicle acceleration sensor is provided with a pivotable unit sensor arm 1. The sensor arm 1 comprises a lower sensor arm section 1a and an upper sensor arm section 1b. The lower sensor arm section 1a and the upper sensor arm section 1b are integrally assembled and mounted on a sensor casing 7, so that they can swing around an arm support pin 3 as a unit.
In addition, an inertial body 6 is positioned on a bearing surface 8. The lower sensor arm section 1a contacts the inertial body 6, and the movement of the inertial body 6 can be transmitted to the upper arm section 1b. The inertial body 6 moves in response to the acceleration (in a minus direction in such a case the vehicle is suddenly stopped), so that if an increase in acceleration greater than a predetermined rate occurs, the inertial body 6 moves into the locked state which is detailed in the following paragraph. Accordingly, the unit sensor arm 1 which contacts the inertial body 6 can swing around the arm support pin 3, and when the inertial body 6 reaches the locked state, the unit sensor arm 1 which transmits the movement of the inertial body 6 rises to the position of a ratchet wheel 4, where the unit sensor arm 1 is interlocked with the ratchet wheel 4 to lock the webbing so as not to be pulled any more.
However, there are two problems associated with the vehicle acceleration sensor of FIG. 1.
The first problem is that it takes considerable time to restore the vehicle acceleration sensor and also the webbing to the unlocked state from the locked state.
In a conventional vehicle acceleration sensor, when a change occurs in the acceleration of the vehicle, a swinging motion and/or circular motion occurs in the inertial body. Even when the vehicle body acceleration becomes zero, considerable time elapses until this motion is damped. For example, suppose that the vehicle acceleration sensor shown in FIG. 1 is in the locked state, but the inertial body 6 is in a state wherein the swinging and/or circular motion can continue on the bearing surface 8 of the sensor casing 7. In this state, when the inertial body 6 exhibits a swinging and/or circular motion, even if the vehicle acceleration is in the unlock stage, the inertial body 6 continuously moves as far as the locked state, so that the ratchet wheel 4 and the unit sensor arm 1 remain in the locked state. Accordingly, the webbing will wind up as a result of the movements of the passenger, but cannot be drawn out, so that the passenger would be subjected to an oppressive sensation.
In addition, when the vehicle acceleration sensor enters the locked state as the result of an incline in the vehicle body, it is delayed in returning to the unlocked state. The inertial body must return to the original position with its own weight in order that the vehicle acceleration sensor moved in the locked state because of the incline of the vehicle returns to the unlocked state, so that a steep hill or the like can cause the inertial body to move in the locked direction and go into the locked state unnecessarily, or the sensor arm is prevented from moving to the unlocked state. As a result, the movement of the sensor arm from the locked state to the unlocked state requires extra time.
The second problem concerns when the device enters the locked state from the vibration of the vehicle body.
Conventionally, the system comprising the inertial body and the sensor casing in the vehicle acceleration sensor for the retractor has an inherent oscillation frequency, characteristic to the bearing surface in a cone-shape or concave spherical shape of the sensor casing in which the inertial body is positioned.
In the vehicle when running on bad roads, oscillations at the inherent oscillation frequency of the vehicle in the longitudinal and lateral directions of the vehicle are produced, and kinetic energy is imparted to the inertial body of the vehicle acceleration sensor from the oscillations to cause vibrations in the inertial body.
In the case where the inherent oscillation frequency of the system including the inertial body and the oscillation frequency of the longitudinal and lateral oscillations are in agreement, even when the acceleration of the oscillations of the vehicle is small (for example, 0.1 to 0.2 G) so that it does not reach the acceleration at which locking should occur in an emergency, the phenomenon of resonance imparts a large amplitude to the inertial body which incurs a continuous swing or circular motion and enters the locked state. Accordingly, the retractor becomes locked, the webbing compresses the passenger who receives an oppressive sensation.
In this way, even though the vehicle acceleration is smaller than the locking acceleration, the retractor sometimes remains in the locked state, which is referred to, in this case, as an overlocked state.
Japanese Laid Open Patent No. Sho 56-93253 reveals a webbing lock device for a retractor in which a lock member is positioned between a sensor arm and a ratchet wheel. Specifically, the lock member engages the ratchet wheel of the retractor through the sensor arm. In this way, the movement of a moving ball, that is an inertial body is transmitted to the lock member.
However, when the lock member engages the ratchet wheel, the arm only rides on the moving ball, so that movement of the moving ball is not adequately prevented. Therefore the same problems occur as outlined for the previous example of the prior technology.